1. Field of the Invention
This invention relates to systems for testing coaxial cable networks. More particularly, it relates to systems for testing the shielding effectiveness of coaxial cable plant so that defects may be identified and repaired.
2. Description of Prior Art
Cable systems currently in use typically allow two way communications between the headend or distribution hubs and many remote points that may be defined as homes and coaxial drop wires to the homes. A headend is a collection point for both upstream and downstream signals. A distribution hub, which is sometimes used in large systems, is an intermediate point between the headend and the fiber nodes where the downstream signals from the headend are split and the upstream signals are combined. For the sake of this patent, the terms headend and distribution hub may be used interchangeably. One frequently employed architecture is hybrid fiber-coax (HFC). Forward direction, or downstream, signals are transmitted from the headend via optical fibers to fiber nodes. At the fiber node, the downstream transmission is converted from an optical signal to an electrical signal. The signal is distributed from the fiber node to a plurality of remote points, which may be homes, via coaxial cable by splitting. Amplification overcomes the losses of the cable and splitting devices. This portion of the network is referred to as a tree-and-branch system. The downstream frequency range is typically 54 to 550 MHz. This downstream system works well because a high quality signal, which consists of many channels, is produced in the headend. The processes of splitting and amplification can produce many high quality replicas of the headend signal. Downstream signals have traditionally been analog television (TV) carriers. Digital carriers, such as digital audio, digital TV, cable telephone, and computer data, are increasingly being transported by the downstream system.
In the return direction, or upstream, signals are transmitted from the remote points in the 5 to 30 MHz frequency band to the fiber node. The same passive devices that acted as splitters for downstream signals act as combiners for upstream signals. At the fiber optic node, the combined upstream electrical signals are converted to an optical signal for transmission to the headend. Forward and return signals typically travel inside the same coaxial cable in opposite directions. The use of diplex filters allows bidirectional travel inside a single coaxial cable. In the fiber optic bundle, forward and return signals commonly travel in opposite directions in different optic fibers.
The upstream system is problematic because noise that is introduced into one branch can contaminate the signals on all branches because the return signals are combined. This problem is commonly referred to as noise funneling. The use of 5-30 MHz for the return band makes the noise funneling problem even more acute, since man-made electrical noise is strong in this frequency band.
It has been discovered that the most common form of return band impairment is high speed bursts of noise that are typically short but powerful. The noise bursts typically last less than 10 microseconds and have most of their energy content concentrated between 5 and 15 MHz. The noise bursts are sometimes powerful enough to distort, or drive, return active devices into a non-linear mode. The common sources of return noise bursts are the switching of electrical devices, such as inductive loads, or motors with brushes. The switching action creates noise bursts which get onto the electrical utility power lines. Electrical utility power lines are commonly connected to the cable lines at bonding points for safety reasons. Some of the energy of the noise bursts on the power lines are transferred onto the coaxial cable lines at the common bonding points. Because of skin effect, the braided shields on flexible cable lines are low resistance paths for burst energy. The noise burst travels on the sheaths of the cable lines until they are radiated away, dissipated by resistive losses, find their way to ground, branch off, encounter a break in the coaxial cable or are otherwise dissipated. At a break in the coaxial shield, some of the burst noise energy enters the inside of the coaxial cable and travels to the fiber node where it causes interference with return transmissions.
Other signal sources, such as broadcast or two-way ham or citizens band radio traffic also present problems if the plant has a shield break.
Breaks in coaxial cable shields inside homes are caused by poor installation practices, mechanical damage, corrosion, and other causes. Another entry point for noise into the cable system is at consumer electronic devices, such as TVs, video cassette recorders (VCRs) and FM band radios. These devices sometimes have poor tuner isolation, so that noise on the cable shield can enter the inside of the coax at the points in the network where these devices are connected. Consumer electronic devices can be frequency selective by allowing noise in one frequency band into the cable at a higher level than noise in other frequency bands
Typical causes of poor shielding integrity in outdoor plant are corrosion, animal chews, screw-on connectors that are not tightened, and housings with loose bolts.
There are multiple failure modes for the shielding integrity of coaxial cable, and shielding effectiveness may be degraded moderately or severely. For the sake of description, any degradation in coaxial cable plant shielding integrity will be referred to as a "shield break".
For a faulty shield upstream transmission problem to occur, two conditions must be simultaneously met. First, a source of undesirable energy must be present on the coaxial sheath. Second, the coaxial sheath must be defective or open at the instant the impairment arrives. Frequently, the coaxial sheath has intermittent continuity and the burst noise source intermittently produces interference. This makes the shield's break point difficult to uncover by observation or passive testing. Additionally, the combining of upstream noise and signals makes it difficult to discover which path the noise burst took to the fiber node.
The traditional method of finding shield breaks is by measuring radiated signal strength from a special narrow bandwidth downstream test signal that is typically located at the high end of the FM radio band. This test method does not adequately find all shield breaks that affect return transmissions. Devices have been recently introduced that can be used to detect shield breaks or shield problems inside homes. One device requires the technician to bring a sensor within a few meters of the break, which mandates entry into the home. Another new device is a reflectometer that allows the technician to stand outside the home and measure the cable's return loss. This device provides distances to discontinuities. This test method does not necessarily provide a distinction between return loss problems caused by bad splitters, missing terminators, consumer electronics devices with poor return loss, staples through cables and shield breaks. If the technician is searching for shield breaks, the other information about conditions inside the cables is confusing and irrelevant.